1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for reducing drag on a towed cable and/or reducing vibrations associated with the towed cable.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill to avoid a dry well.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic detectors provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The vessel 10 also tows a seismic source assembly 16 that is configured to generate an acoustic wave 18. The acoustic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the acoustic wave. The reflected acoustic wave 24 propagates upward until it is detected by a detector 26.
The streamers 12 are shown in FIG. 2 spreading over a predetermined area. This is called the seismic spread. In order to maintain the plural streamers 12 substantially parallel and at equal distance from each other, various front-end gears are used. Streamers 12 are spread out to a desired width to provide measurements of the geological conditions over an acquisition area.
An example of a front-end gear 30 is shown in FIG. 2. The front-end gear 30 is provided between the vessel 10 and the various streamers 12, and this gear is configured to achieve the desired positioning for the streamer heads. FIG. 2 shows the front-end gear 30 to include cables 32 connected between the vessel 10 and deflectors 34. A cable is used herein to mean a rope, a metallic cable, a synthetic cable, a wire, etc. Deflector 34 is a structure capable of generating the necessary lift when towed to keep the streamers deployed in the transverse direction with respect to the sailing line of the towing vessel 10. Spacers 36 are attached to the cables 32 for distributing the lift force among them in order to obtain a substantially linear profile for the position of the streamer heads. The spacers 36 may be cables, wires, etc. Other cables may be present in the seismic spread and/or in the sources but, for simplicity, these other cables are not shown here.
However, all these cables, including the cables 32 and the spacers 36, generate drag when towed underwater by the vessel 10. The drag generated by all these cables may be substantial. In addition, when a cable is towed underwater and extends substantially perpendicular to a traveling distance, the cable oscillates or vibrates. Thus, the cables use the engine power of the vessel 10 and also introduce noise (due to the vibrations) that is undesirable because the streamers 12 will record seismic data and the noise produced by the cables.
A solution to the above problems is to add fairings to the cables. Conventional fairings are permanently attached to the cables by the manufacturer. However, this solution has the limitation that fairings damaged during the operation cannot be easily replaced. In addition, the operator of the cable cannot move the fairings along the cable to accommodate various seismic survey geometries.
U.S. Pat. No. 4,365,574 (herein the '574 patent) discloses, as shown in FIG. 3, which corresponds to FIG. 1 of the '574 patent, a fairing 40 that includes a nose 42, a tail section 43 and a pair of flanks 44. The fairing 40 can be attached to a cable 46 to reduce its drag. The way the fairing 40 is attached to the cable 46 is as follows. A spreader bar 48 is used to pry apart the tail 43. The spreader bar is thus inserted into a slit 50 of the tail section 43 and when the size of the slit 50 is large enough, the cable 46 is slid in or out of the fairing 40.
However, the process is cumbersome and labor-intensive because one person needs to hold the fairing 40 and another one has to pry open the slit 50 while either the first or the second person has to also handle the cable 46. Thus, there is a need to design a new fairing that can easily be attached or detached from a cable.